1. Technical Field
The present invention relates to an echo attaching circuit and an audio device using the same and, particularly, to an improvement of a circuit for producing an echo signal from an audio signal and attaching the echo signal to the audio signal, that is, an echo attaching circuit to be used in an audio device such as Karaoke device, component stereo device, radio cassette device or VTR, etc.
2. Background Art
Almost all of the recent audio devices are equipped with the so-called Karaoke function with which an audio input from a microphone can be mixed or synthesized with other reproduced signal, etc., and output externally. For such function, an echo attaching function for attaching an echo effect is indispensable for which the echo signal must be internally produced. Echo signal is usually produced by delaying an audio signal which is obtained by converting an audio information such as voice and/or music into an electric signal by a certain constant time. In order to enhance the echo effect, the echo attaching circuit operates to attenuate the audio signal and the echo signal and feeds back them to the side of the audio signal to produce a signal which is a mixture of the audio signal and the fed back signal, and the delaying and mixing operations are repeated for thus produced signal.
FIG. 5 shows an example of a construction of a conventional echo attaching circuit. In FIG. 5, an echo attaching circuit 8 includes a waveform synthesizing circuit 1, a low-pass filter (LPF) 2, a delay circuit 3 using a BBD (Bucket Brigade Device), a low-pass filter 4, attenuators (ATTs) 5 and 6 and a waveform synthesizer 7.
An audio signal A input to the echo attaching circuit 8 is synthesized or combined in the waveform synthesizer 7 with a signal which is obtained by attenuating an echo signal B down to a suitable level by the attenuator 6, resulting in an audio signal C to which the echo is attached. The audio signal C is amplified and output from a loudspeaker as sound or recorded on such as tape by means of a recording circuit.
The echo signal B is produced by a feedback loop including the attenuator. That is, the echo signal B output is attenuated by the attenuator 5 down to a suitable level and fed back to an input side in which it is mixed with the audio signal A by the waveform synthesizer 1. Then, the synthesized signal is passed through the low-pass filter 2, resulting in a signal D. A delay signal E is produced by delaying the signal D by a constant time by the delay circuit (BBD) 3 and the echo signal B is produced by passing the signal E through the low-pass filter 4. The low-pass filters 2 and 4 are provided in an upstream and a down stream of the BBD 3 to prevent an aliasing distortion by allowing components whose frequencies are lower than a half of a sampling frequency of the BBD 3 to pass through. By removing noise component of the delay signal E by means of the low-pass filter 4, the delay. signal E becomes the echo signal B. The audio signal becomes the echo signal by further attenuating the echo signal E thus produced by delay and this is circulated while being attenuated and the echo effect is obtained by repeating this procedure.
The BBD 3 having construction for delaying an analog signal itself is noise-sensitive and easily degraded with time. Therefore, its accuracy and reliability are insufficient. On the other hand, demand of improvement of function of the audio device by users or customers is becoming severe more and more, and, for example, a dynamic range of about 90 dB is required for an audio signal with echo signal in Karaoke function.
In order to satisfy such demand, there was provided a digital memory delay circuit composed of an A/D converter, a controller, a memory and a D/A converter, etc. The digital memory delay circuit operates to delay an audio signal by storing it in the memory thereof as a digital data and reading it after a constant time lapses. In such delay circuit, however, the number of conversion bits of the A/D converter as well as the D/A converter becomes large. Application Ser. No. 8,008,712 assigned to the assignee of this application discloses an invention related to a delay circuit in which an audio signal is delayed by an A/D converter and a memory, with smaller number of bits. FIG. 4 shows a construction of the delay circuit disclosed in application Ser. No. 8,008,712, schematically.
In FIG. 4, a reference numeral 10a depicts a circuit board of an echo attaching circuit to be mounted on the side of a main body on which a main circuit of an audio device is mounted. On the circuit board 10a, a compression circuit 11 for logarithmic compression transformation of an analog input voltage signal, a digital memory type delay circuit 12 which delays an audio signal by converting it into a digital signal and storing it in a memory as mentioned previously and an expansion circuit 13 for exponential transformation (as an inverse transformation with respect to logarithmic transformation) are provided. A reference numeral 20a depicts a circuit board of the echo attaching circuit to be mounted on a front panel of the audio device, that is, the so-called operation panel. On the circuit board 20a, a volume controller 21a for echo regulation which serves as an attenuator for setting a feedback rate of a loop and which may comprise a variable resistor and a volume controller 21b for reverberation regulation which serves as an attenuator for setting reverberation effect and which may comprise a variable resistor are mounted.
FIG. 5 shows a modification of the echo attaching circuit shown in FIG. 4, in which the feedback rate of the loop and the reverberation effect are simultaneously set by a single volume controller 21. The modification is not disclosed in the previously mentioned U.S. Application.
An original signal A input to the echo attaching circuit 10a (10) is synthesized with a signal B' which is obtained by attenuating an echo signal B to a suitable level by the volume controller 21b (21) and output as an audio signal C having echo attached.
The echo signal B is produced by a feedback loop including the volume controller 21a (21). That is, the echo signal B output is attenuated to a suitable level by the volume controller 21a (21), returned to an input side as a feedback signal B" and synthesized with the original signal A, resulting in a signal waveform including the echo signal synthesized with the original signal A. The resultant signal is compressed once by the compression circuit 11 and then delayed by a constant time by the delay circuit 12, resulting in a delay signal D. The delay signal D is expanded by the expansion circuit 13, resulting in the echo signal B which is synthesized again with the original signal.
By attenuating the echo signal B produced by delay and feeding back it, the reverberation effect is produced. By adding this to the original signal at a certain ratio, the audio signal C having echo attached thereto whose amounts of reverberation and echo are variable is obtained.
In an audio device having the echo attaching circuit mentioned in application Ser. No. 8,008,712 or other conventional echo attaching circuit, a user can regulate amounts of echo and reverberation by means of a volume controller composed of a variable resistor and provided on a front panel thereof.
Consequently, the echo signal B produced in the circuit on the circuit board reciprocates between the circuit board on the main body of the audio device and the control board provided on the front panel repeatingly until it becomes the signal B' through attenuation by means of the volume controller 21. Thus, the echo signal produced as an analog signal becomes easily influenced by noise, compared with the original signal.